1. Field of the Invention
The present invention relates to a device for detecting the height of a bonding surface used in a wire bonding machine.
2. Prior Art
Conventional detectors for detecting the height of a bonding surface in a bonding machine for semiconductors include, for example, the apparatus described in Japanese Patent Publication No. 57-58054.
This apparatus comprises a bonding tool through which a bonding wire is passed, a bonding arm which holds the bonding tool, a vertically moving block which moves up and down and to which the bonding arm is fastened so that the bonding arm is free to pivot, a first contact fastened to the bonding arm or to a lever which is attached to the bonding arm, and a second contact which is fastened to the vertically moving block so that the second contact faces the first contact. An electrical current is applied to the two contacts, and the position at which the bonding tool contacts the bonding surface is detected by means of the ON-OFF action of the contacts.
Specifically, when the bonding tool is lowered in the Z direction (the perpendicular direction) so that the bonding tool contacts the bonding surface, the contacts are opened (separated). Contact of the bonding tool with the bonding surface is confirmed by the opening and closing of the contacts, and a standard programmed amount of "sink-in" is determined.
In the abovementioned prior art, the bonding surface is detected by the opening and closing action of the contacts. As a result, the following problems arise:
When the contacts open or close, they generate minute amount of chatter. As a result, confirmation of the opening of the contacts may be delayed, or errors in operation may occur. This causes unstable "sink-in" to occur.
Furthermore, the material used for the contacts must have good electrical characteristics, (such as a material formed by applying gold plating to an alloy of silver and titanium or an alloy of beryllium and copper, etc.) which accurately detects the opening and closing of the contacts by causing a minute electric current to flow therethrough. Accordingly, the contact material is inferior in terms of wear resistance. As a result, not only does stable detection become impossible as the frequency of opening and closing of the contacts increases, but mechanical discrepancies may also occur.